The proposed study will give information about the localization, binding and mobility of cholesterol in the erythrocyte membrane. Work done to date in this project has shown that the cholesterol molecules located in the inner and outer halves of the bilayer in the erythrocyte membrane do not form kinetically distinct pools, namely that transbilayer movement of cholesterol occurs in this system. Removal of cholesterol from red cells can be accomplished by incubating cells with phospholipid vesicles or with plasma of lowered free cholesterol content. Studies are proposed to determine whether all of the cholesterol molecules in the membrane are equally available for removal or whether there exists a pool of cholesterol which cannot be removed. Abnormal erythrocytes containing excess cholesterol such as spur cells also will be studied to determine whether the excess cholesterol is segregated from the rest of the membrane cholesterol. X-ray diffraction studies will be carried out to obtain structural information about erythrocytes of modified cholesterol content. BIBLIOGRAPHIC REFERENCES: Poznansky, M. and Lange, Y. Non exchangeable 3H-cholesterol in metabolically labeled intact mouse erythrocytes. Abstract- Biophys. J. 17, 72a (1977). Lange, Y., Cohen, C.M. and Poznansky, M. Transmembrane movement of cholesterol in human erythrocytes. Proc. Nat. Acad. Sci. USA (1977), in press.